The present invention relates generally to lead frames for use in integrated circuit packaging and more particularly, lead frames having flow diverters for bifurcating the flow of encapsulating material during the molding of a package for a semiconductor device.
Semiconductor dies are created from a silicon wafer through the employment of various etching, doping, and depositing steps that are well known in the art. Ultimately, the semiconductor die may be packaged by forming an encapsulant around the semiconductor die so as to form an "integrated circuit package" having a variety of pinouts or mounting and interconnection schemes. Plastic is often utilized as an encapsulant. Integrated circuit packages that utilize plastic as an encapsulant are less expensive then other packaging options and provide performance and reliability that is acceptable for a number of different applications.
An array of electrical conductors called a "lead frame" forms both the support structure for a packaged semiconductor die as well as the conductive paths between a semiconductor die and external circuitry consisting of a set of lead frame fingers otherwise known as leads. On one end, each lead is electrically connected to the semiconductor die by means of a bonding wire. For instance, in the case of a lead on die package, each lead is designed to align with and connect to one of a series of connection, or bond, pads that are located on the face of a semiconductor die. These bond pads are the points at which all input and output signals, as well as power and ground connections, are made for the semiconductor die to function as designed. The other end of the leads, being external to the integrated circuit package and extending from the leads are further connected to external circuitry or alternatively may be connected to an intermediate package such as a hybrid circuit or multichip module.
As illustrated in FIGS. 1(a) and 1(b), a conventional lead frame 10 generally includes an array of electrically conductive leads 11, a support pad 12 which is downset relative to the leads 11, and lead frame body 18. The support pad 12 is composed of a die attach area 13 (which supports an attached semiconductor die 20), a support hole 15, support arms 17 (used to support the die support area during handling), and flow holes 14. Conventional lead frames are typically formed from a single metal strip. The lead forming step itself may be either an etching process or a stamping process. In these processes the areas between the desired features are removed through either stamping or etching. In the etching process, a mask of the pattern of the desired features is laid over a metal strip. The exposed areas are then etched away creating the features desired in the metal strip. The stamping process usually consists of stamping out the desired patterns in the lead frame blank. Multiple stamp tool punches, shaped in the form of the respective patterns punch out the respective features in the lead frame blank. A pressing tool then forces the support pad 12 (including the die attach area 13, support hole 15 and flow holes 14) to become downset relative to the leads 11 and the body of the lead frame 18.
Fabrication methods for semiconductor die packages are well known to those skilled in the art of semiconductor packaging and are generally straightforward. A semiconductor die 20 is attached to the die attach area 13 and associated bond pads 22 are electrically connected to the lead tips 16 with bonding wires 21. An encapsulant material is then used to surround the inner portion of the lead frame, including the attached semiconductor die 20 and a substantial portion of the leads 11. The excess metal 18 that supported the entire lead frame 10 is then trimmed away to free the leads 11 from each other. The end product is a packaged semiconductor device.
One of the disadvantages with this method of packaging a semiconductor device is due to the fact the encapsulant material is applied in the form of a primary stream 30 which due to the downset nature of the support pad 12, preferentially surrounds the upper surface 23 of the die attach area 12 which includes the attached semiconductor die 20. As the encapsulant material is applied, an upper flowstream 31 impinging on the upper surface 23 of the die attach area 12 and a separate lower flowstream 32 of encapsulant impinging on the lower surface 19 of the die attach area 13 are created, the flow rate of the upper flowstream 31 being substantially greater then the flow rate of the lower flowstream 32. The difference in flow rates causes a pressure gradient to develop between the upper surface 23 of the die attach area 12 and lower surface 19 of the die attach area 13. This pressure gradient causes 1) any gas bubbles to become entrapped beneath die attach area 13 since the faster moving upper flowstream 31 prevents the bubbles from escaping which results in a multitude of voids in the region beneath die attach area 13 after final curing of the encapsulant, and 2) the die attach area 13 to be forced out of planar alignment with the lead frame body 18 and the leads; this misalignment adversely affects the integrity of the wires 21 attached to the bond pads 22 on the semiconductor die 20 by mechanically stressing both the bond pad/bond wire junctions and the bond wires themselves. The non-uniformity of the thickness of the encapsulant on the underside of the die attach area 19 adversely affects both package heat transfer characteristics and package long term reliability. In order to mitigate the problems caused by this pressure gradient, flow holes 14 are used to increase the flow rate of the lower stream 32 relative to the upper flowstream 31 by diverting a portion of the primary flow 30 of encapsulant to the lower flowstream 32 in an attempt to equalize the flow rates. However, in some implementations, the flow holes 14 alone cannot divert a sufficient amount of encapsulant material to the lower flowstream 32 to prevent the creation of the pressure gradient with the abovementioned adverse effects.
The present invention provides a means for adjustably increasing the flowrate of he lower stream 32 of encapsulating material and thus reducing or eliminating the pressure gradient and its associated problems as described. The presently disclosed invention is fully compatible with the standard manufacturing process for lead frames.